1. Technical Field
The subject invention relates to uninterruptible power supply (UPS) systems generally, and more specifically, to isolated-parallel rotary UPS configurations.
2. Background
Uninterruptible Power Supply (UPS) systems are often used to protect critical facility electrical loads from power disruptions where loss of equipment functionality cannot be tolerated. UPS systems typically incorporate some form of short term power storage to support “no-break” electrical loads while a backup power source is engaged and connected to the loads. The backup power is very often supplied from one or more diesel-fueled engine-generators located on the facility premises.
Diesel UPS Systems, for example, rotary Uninterruptible Power Supply Systems using flywheel-type energy storage coupled with standby diesel engines, are a popular form of power backup due to their compact size and lack of large chemical batteries. However, they can have some drawbacks, including, for example, the short ride-through time of flywheels compared to batteries. More significantly a flywheel-based UPS typically requires a one-to-one pairing of a flywheel with a diesel engine, usually involving a mechanical clutch mechanism between the constantly rotating UPS machine and the mostly idle standby engine. Therefore, a failure of a diesel can also mean failure of the associated UPS.
To offset these potential drawbacks, Diesel UPS Systems are often connected into paralleled groups for both capacity and redundancy. Parallel groups typically have two or more UPS systems, or modules, connected to a common output bus, and may have extra modules for redundancy. This is referred to as paralleled module redundancy, or parallel-redundancy. Recent designs for “upper tier” facilities go beyond parallel-redundancy and incorporate multiple parallel groups for the critical loads in which critical parallel busses as well as modules are duplicated for redundancy. Such group redundancy, often referred to as “N+N redundancy,” provides two or more separate configurations capable of supporting all critical loads with one parallel group out of service. Since the engines in a typical Diesel UPS System share the same redundancy level as the flywheels, parallel group redundancy in Diesel UPS Systems can become quite costly compared to battery-type UPS systems backed up by a separate, standby engine-generator system that may be installed with only module redundancy, or “N+1 redundancy.”
In order to provide more cost competitive solutions and still provide system output bus redundancy, Diesel UPS Systems, as well as most other types of UPS systems, can be arranged in isolated-module redundant configurations in which a dedicated redundant module backs up two or more separate load carrying modules. The larger the total critical load is in relation to the size of the module or system employed, the more economical it is to configure the equipment as an isolated-module redundant N+1 configuration versus a group-redundant N+N configuration. Arranging large numbers of UPS systems into independent modules instead of massively paralleled groups has the added benefit of reducing fault current levels at the loads, and limiting the effects of faults to smaller portions of the total load.
However both parallel group redundant and isolated-module redundant configurations rely on fast switching mechanisms to transfer critical load from a failed group or module to the designated redundant group or module. Also, isolated-module redundant configurations are susceptible, under certain kinds of stimulation, to the risk of overloading the redundant module. There have been cases documented for both Diesel UPS System and battery-type UPS systems where several primary modules simultaneously reacted to a mutual disturbance and transferred their critical loads to the redundant module, crashing it and the loads. Furthermore, isolated-module redundant configurations with independent modules serving independent loads, as opposed to paralleled modules with a combined load, can result in load imbalances on the modules, and possible overloading or under-utilization of certain modules. Accordingly, a configuration of UPS systems to overcome the above deficiencies is needed.